1. Field of the Invention
The present invention relates to velocimetry, and more specifically, it relates to the use of a velocity interferometer capable of using unlimited bandwidth illumination.
2. Description of Related Art
Measuring the velocity of an object remotely through the Doppler shift of reflected wave radiation (electromagnetic waves, sound) is an important diagnostic tool in a variety of fields in science and engineering. In shock physics, velocity interferometry is an important optical technique measuring velocities of impacted targets, typically moving 1-10 km/s. In law enforcement and meteorology, other kinds of Doppler velocimeters using microwaves measure velocities of automobiles and raindrops. And in medicine using ultrasound, Doppler instruments detect the motion of blood in vivo. All these velocimeters require coherent quasi-monochromatic illumination, which is generally more expensive and lower power than broadband incoherent sources, for any kind of wave radiation. For example using light, for the single interferometer velocimeter, the illumination was previously restricted to lasers operating in a single longitudinal frequency mode, which reduces their power. For the double interferometer velocimeter (U.S. Pat. No. 4,915,499), the illumination was restricted to less than 4 nm. In comparison, visible light (red to blue) has a bandwidth of approximately 200 nm. (The limited bandwidth was calculated by the authors of that patent in an article "Multiple-line laser Doppler velocimetry" by S. Gidon and G. Behar, Applied Optics Vol. 27, p2315-2319, 1988, on page 2317, below Equation 11.)
The invention and the prior art velocimetry discussed here is characterized by a single illuminating beam striking the target. This is distinguished from another widely used velocimetry technique using two intersecting laser beams in a transparent fluid, in which scattering particles carried by the fluid pass through a standing wave grating created by the intersecting beams. That technique is commonly called LDV, for laser Doppler velocimetry. (The title is not helpful in distinguishing the different kinds of prior art, since they all are based on Doppler shifts and almost all on laser illumination.)
The prior art of velocity interferometry (single beam striking the target) can be classified into kinds using a single interferometer, or two interferometers before and after the target. The former is much more common. For use with light, single interferometer velocimeters are distinguished by the kind of interferometer. Those using monochromatic superimposing Michelson interferometers are called VISARs (velocity interferometer system for any reflector), and those using non-superimposing Fabry-Perot interferometers are called Fabry-Perot velocimeters. The latter are non-superimposing because of their design, (and their method of use depends on the non-superimposing nature through the creation of off-axis fringes which shift in angle with velocity.) The definition of superimposing will be defined later. A double interferometer using two non-superimposing Fabry-Perot interferometers is described in U.S. Pat. No. 4,915,499.
The prior art either uses single interferometers, or double interferometers lacking in the proper superimposing quality. The prior art did not recognize that in order to use unlimited bandwidth incoherent illumination, two achromatic superimposing interferometers are needed, before and after the target. (The two can be optionally conveniently accomplished with one set of interferometer optics by retro-reflecting the light from the target.) Furthermore, the superimposing designs presented in the present invention are achromatic, whereas the prior art of single superimposing interferometers (VISARs) suffered chromatic aberrations which limit their use to monochromatic light.
The distinction between the superimposing quality of the present invention and the non-superimposing quality of the prior art is important to understand, and is crucial in creating many of the beneficial capabilities of this invention. The non-superimposing velodmeter cannot use all the illuminating power available from a non-directional broadband source, such as an incandescent lamp, due to limits on bandwidth or numerical aperture. It cannot form an imaging velocimeter. It cannot use unlimited bandwidth illumination from an uncollimated source. This in turn prevents many desirable capabilities that come from a wide bandwidth, such as simultaneous multiple velocity detection, unambiguous velocity determination, independence from target and illumination color, and lack of speckle.
The velocity sensitivity of all the velocity interferometers discussed, single or dual interferometer, superimposing or non-superimposing, broad or narrowband, all have the same fundamental relationship between fringe phase shift for a specific color, and target velocity. This will be described now. The additional advantageous that come with broadband illumination will be discussed later.
The single interferometer velocimeter consists of a monochromatic source illuminating the target, and an interferometer analyzing the reflected radiation. If the interferometer is a Michelson design, it has a fixed delay .tau. between its two arms. In the case of a Fabry-Perot interferometer, .tau. describes the delay between multiple output pulses produced from a single input pulse, if the illumination was considered hypothetically to be a stream of independent pulses. The nonzero value of the delay converts small Doppler shifts of the reflected light spectrum into fringe shifts in the interferometer output. Fringes denote the fluctuations in time-averaged output intensity which vary sinusoidally with target velocity v. There is a proportionality .eta. between the target velocity and fringe phase shift for a specific wavelength of illumination described by ##EQU1## and .lambda. is the average wavelength of light. For example, to measure highway velocities in green light, .eta. should be of the order 10 m/s, which requires c.tau.=8 m, where c is the speed of light. (Delays are conveniently described either by duration .tau., or the distance light travels in that time c.tau..) Equation (1A) neglects dispersion in the glass optics inside the interferometer, assumes v/c&lt;&lt;1 and that light reflects normally off the target. The velocimeter can measure target displacement or velocity, depending on the time scale of the recording system. If the detector response time is very fast, faster than the interferometer delays, but still slower than a system coherence time L.sub.sys /c, then the fringe shift .DELTA.f is related to a displacement of the target according to ##EQU2## where d is component of the target motion toward the source and detecting interferometers. This is not the typical situation. Typically the detector or recording equipment response time is slower than the delay time t. In this case, the fringe shift is related to velocity according to ##EQU3##
The velocity component of the target toward the velocimeter is obtained from the fringe phase shift .DELTA..phi. by ##EQU4##
In previous velocimeters, the coherence length (.LAMBDA.) of the illumination must be as large as c.tau. in order to produce fringes with significant visibility. This severely restricted the kind of light source which could be used. The .LAMBDA. of white light (.about.1.5 .mu.m) was insufficient. Previously, lasers were the only light sources used because their coherence length could be made sufficiently long when operated in a single frequency mode. However, in this mode the output power is low.
The low output power restricted the applications of optical velocimetry. Typical laboratory measurements in shock physics were limited to measurement of velocity at a single point on the target. These experiments, and many other laboratory and industrial applications can be greatly improved if sample velocity could be measured simultaneously at more than one point, such as over a line or an area. For example, to measure the velocity field over an area at a single moment in a wind tunnel, or to measure the complex motion of an exploding non-symmetrical object.
However, measuring velocity over a line or an area in a snap-shot requires orders of magnitude more power than what can be provided by single mode lasers. Amplifiers can be used to boost laser power, however, these are expensive and bulky. Furthermore, velocimetry of a remote object through a telescope in the field demands orders of magnitude more power, since reflected light intensity is weak. Previously, the velocity can be measured over an area by scanning a single point measuring beam. However, this is not appropriate for measuring complex dynamic events such as turbulence, which grow on a time scale faster than the scanning can be completed. In contrast, the invention we describe can measure velocities simultaneously over an area, taking advantage of the velocimeter's imaging capability, and the higher power available from inexpensive and compact broadband incoherent sources. These sources are not suitable for a conventional velocity interferometer due to their incoherence. Even the prior art double Fabry-Perot velocimeter can only use but a fraction of the available power because its 4 nm bandwidth limit is approximately 1/50th of the visible spectrum. Secondly, it cannot be used in an imaging mode because its non-superimposing Fabry-Perot creates fringes as a function of ray angle off the central axis. In contrast, the superimposing Fabry-Perot described in this application creates a fringe phase which is constant over a range of ray angles.
For the purpose of illuminating a target over a wide area, the directional quality of a laser beam is not a comparative advantage, since the laser light must be dispersed to cover the area, and the target will usually randomly scatter the reflected light in all directions. For example, a $50,000 argon laser operating in single-frequency mode only produces approximately 1 watt of power. In contrast, a flashlamp costing a $1000 dollars can create perhaps a 100,000 watts. If the superimposing interferometers are achromatic and have a large numerical aperture capability, then a large fraction of the ray angles and colors of the flashlamp can be utilized for white light velocimetry. Thus, for illuminating an area, more illuminating power can be obtained from the flashlamp than the laser, and for less expense.